The present invention relates generally to dynamic braking of an electric motor and more particularly to improved method and apparatus for providing dynamic braking of an induction motor operated by a controlled current inverter system.
Dynamic braking, broadly speaking, is a well-known method of electrically braking an electric motor. It is sometimes used alone but often finds use in conjunction with other braking methods such as regeneration as a supplement to such other braking method when for some reason, the motor power source is unable to accept energy from the motor. In essence, dynamic braking involves the insertion of a resistive element into electrical circuit with the motor such that the motor, now acting as a generator, generates energy which is dissipated in the resistive element.
In the aforementioned U.S. Pat. No. 4,230,979 there is described and claimed a motor control system of the controlled current inverter type which includes dynamic braking and in which a controlled direct current (d.c.) power converter furnished electric power by way of a d.c. link, including a selectively insertable dynamic braking resistor, to an inverter of controllable frequency. In this application, the motor control system is responsive to a torque reference signal. Three control paths responsive to the torque reference signal act to control the motor. A first path is employed to control the d.c. converter and hence the d.c. current within the link. A second path is used to control the frequency of inverter operation to thus control the angle between the motor flux and the motor current (referred to as the air gap power factor angle, .theta.). The third path is responsive to the torque reference and instantaneous motor flux and serves as a trim function to each of the other paths.
In accordance with the teaching of that U.S. Patent, when dynamic braking is desired, the d.c. converter is short-circuited. At essentially the same time, a speed reference which is used to generate the torque reference is forced to zero and the torque reference is also forced to zero. The dynamic braking resistor is then rendered effective and the torque reference signal again allowed to become effective.
While this system in its basic form is adequate for many situations, it has the potential of becoming unstable in certain instances in that an improper level of flux could exist which, in turn, could result in equipment damage or improper braking action. If, for example, the flux were too high when dynamic braking resistor is inserted, the motor current could become excessive and equipment damage could result. If the flux is too low when the dynamic braking resistor is inserted, the flux could collapse completely and little or no braking action would occur.